Capacitors are used in a wide variety of electrical circuits, for example in relatively high voltage AC power systems (such as the common 110-volt systems) and in relatively low voltage (e.g., under 50 volts) DC systems frequently encountered in printed circuits and the like. Important factors which must be considered in the manufacture of such capacitors are volumetric efficiency, temperature of operation, dissipation factor, especially in AC systems, and behavior upon failure.
The development of electronic devices and circuits of reduced size has led to a need for significantly smaller capacitors having increased volumetric efficiency, or capacitance per unit volume. The monolithic capacitor has been used for such applications.
A monolithic capacitor is one in which the layers of electrodes and dielectric are bonded together in a unitary structure as opposed, for example, to a metallized film capacitor in which self-supporting films are rolled or wound into the capacitor form. A miniaturized capacitor is one of very small dimensions, so as to be suitable for microcircuitry. Small overall size could denote low capacitance of little practical value, except that the thickness of the intervening dielectric layer inversely affects the capacitance between adjacent electrodes, and the number of electrode pairs and dielectric constant of the dielectric directly affects capacitance. Therefore, as a matter of basic capacitor theory, a capacitor having very thin dielectric layers, and many pairs of electrodes or a given capacitor with a dielectric having a high dielectric constant could have substantial capacitance despite being of miniature size with the active area of the electrodes being quite small.
One such type of monolithic multi-layer capacitor is described in application Ser. No. 562,779, cross-referenced herein. That capacitor has a capacitively active section, and two electrode joining sections, each separated from the active section by a sloping section. The capacitor includes a first and second set of electrode layers interleaved with one another, each layer of each set having an active area extending through and contributing to the capacitively active section of the capacitor in a stacked and spaced apart relationship with the active areas of all of the other layers. The electrode layers are joined at the margin in stacked electrically contacting relationship and each layer has a sloped portion between its active area and its margin which contributes to a sloped section of the capacitor. A dielectric coating is in contact with and between each adjacent electrode pair. The dielectric coating is a substantially uniform thickness in the capacitively active section and tapers to zero thickness through the sloping section.
The volumetric efficiency of a capacitor, including the monolithic multi-layer capacitor described above, is normally measured in terms of capacitance per unit volume. Generally, high efficiency is desirable, with values of at least about one-tenth (0.1) microfarad per cubic millimeter for a 50 VDC rated unit being preferred.
A wide variety of dielectrics are known and used in capacitors presently commercially available. Such dielectrics include polyethylene terephthalate, (more commonly known as Mylar.RTM.--a trademark of Dupont), polycarbonates, polysulfones, polypropylene. The dielectric constant of these materials is generally less than about 4. More recently a wide variety of polyfunctional acrylate polymers and mixtures thereof useful as dielectrics were described in the patent applications cross-referenced above, particularly U.S. Pat. No. 4,499,520. The dielectric constant of these materials is also generally on the order of less than about 4.5 or so.
As a matter of basic capacitor theory, when such materials are used as dielectrics, the volumetric efficiency of the capacitor may be increased by reducing the thickness of the dielectric layer and/or by increasing the number of electrode pairs, both of which may have limits depending upon the capacitor type and its end use. Thus there remains a need for increasing the volumetric efficiency of a capacitor of a given size and structure by providing materials whose dielectric constant is higher than the dielectric constant of presently commercially available materials.
Accordingly, it is a principal object of the present invention to provide a capacitor having higher capacitance per unit volume. A related object is to provide a capacitor having a dielectric whose dielectric constant is higher than dielectrics heretofore provided.
It is yet another object to provide a monolithic multi-layer capacitor having higher capacitance per unit volume.
It is a further and more specific object of the invention to provide a capacitor which is well suited for direct current applications.
These and other objects and advantages of the invention will become apparent upon reading the detailed description. While the invention will be described in connection with the preferred embodiments, it will be understood that I do not intend to limit the invention to those embodiments. On the contrary, I intend to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.